Conventionally, an imaging device using an image pickup device, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), has been known. In such an imaging device, an image of an object is formed on an image pickup device and electronic signals from the image pickup device are processed in order to create image data that may be recorded or further processed to provide an image for viewing. Miniaturization of the image pickup devices has been progressing in recent years in order to make the entire imaging device smaller.
In particular, the miniaturization of module cameras for inputting picture images in cellular phones and digital still cameras (hereinafter referred to as digital cameras) has been remarkable in recent years. Conventionally, the imaging lens used in small imaging devices has been a small, single lens component in order to achieve the desired small size and portability desired. Additionally, in recent years, the performance capabilities of image pickup devices has been improved, and small image pickup devices with higher pixel densities have been developed without increasing the size of the image pickup device. Associated with the realization of higher pixel density, higher optical performance of the imaging lens used with the image pickup device is demanded. However, a small, single lens component does not satisfy the optical performance requirements needed.
Recently, in order to obtain tolerable optical performance for image pickup devices having a high pixel density, consideration has been given to increasing the number of lens components of the imaging lens. However, while increasing the number of lens components enables improvement of the image quality, it is disadvantageous from the standpoint of increasing the overall length of the imaging lens, and may result in the small size and portability of the imaging device being lost. Conventionally, an imaging lens with a high optical performance that is suitable for mounting in a small, image pickup device having a high pixel density has not been fully developed in terms of satisfying the desired miniaturization requirements and providing a satisfactory image quality.
Furthermore, as an imaging lens for an imaging device, the optical performance requirements may not relate solely to the imaging lens itself but may depend on the characteristics of the image pickup device. For example, in the case of using an image pickup device such as a CCD, in general it is desirable that light rays generally are incident onto the image surface of the image pickup device nearly normal to the surface. In other words, it is desirable to secure a telecentric property at the image plane. Therefore, it is desired to develop an imaging lens with various performance capabilities that match with the characteristics of an image pickup device with which the imaging lens may be used.